Process for removing harmful substances from liquid carbon dioxide and apparatus for the performance thereof

ABSTRACT

A process for removing harmful substances from a gas steam which includes carbon dioxide and substances of value, such as at least one of the gases hydrogen H 2  carbon monoxide CO, nitrogen N 2  or noble gases, and harmful substances such as a substance from the group of mercury, sulphur, mercury compounds or sulphur compounds, wherein a carbon dioxide condensation is performed in order to obtain liquid carbon dioxide, adsorptive removal of the harmful substances from the condensed carbon dioxide is performed to remove the harmful substances from the carbon dioxide. A process temperature of less than −30° C. but great than −70° C. is maintained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2010/060335, filed Jul. 16, 2010 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2009 035 389.5 DE filed Jul. 30, 2009. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a process and to an apparatus for separating harmful substances from a carbon dioxide stream. Compounds of sulfur or mercury are examples of harmful substances.

BACKGROUND OF INVENTION

On account of the so-called greenhouse effect and the global temperature rise associated therewith, the emission of greenhouse gases into the atmosphere is subject to increasing criticism. A considerable proportion of the greenhouse effect is attributed to carbon dioxide CO₂ which is produced during the combustion of fossil fuels.

There is currently a consensus across society that separating out carbon dioxide from power station waste gases may counteract an increase in the greenhouse effect. For this reason, corresponding further developments are directed at making possible plants having low levels of carbon dioxide emissions or even none at all. Accordingly, the latest concepts for power stations having low levels of carbon dioxide emissions or which are free of carbon dioxide emissions are at the development stage. All of these concepts pursue the common objective of separating the carbon dioxide as completely as possible and at a high level of purity with minimal energy consumption.

Here the focus is on the separation of the carbon dioxide in the first instance. In addition, however, further undesirable substances are present which occur depending on the type of fuel and which need to be removed. Sulfur, mercury or their compounds are the most commonly encountered substances in terms of quantity in this context.

The latest power station concepts include general conditions which in each case provide the most advanced techniques for separating harmful substances according to the current prior art.

According to the prior art, harmful substances which accumulate in power stations, such as sulfur or mercury compounds, are removed directly from the gas phase. Thus, hydrogen sulfide, for example, is separated by means of gas scrubbing. This entails routing the gas stream through an absorber in which a liquid medium absorbs the harmful substances. Liquid absorber media are for example aqueous alkanolamine solutions, specifically aqueous methyl diethanolamine, or for example cold methanol, employed in the Rectisol process. Such processes using liquid absorber media are known and established in various technologies.

Disadvantages in the prior art consist primarily in the fact that gas scrubbing processes involve an increased energy requirement because they may be associated with process steps such as compression and/or cooling.

The regeneration of absorber media is accomplished by means of distillation, with which a substantial degree of energy consumption is associated.

Furthermore, processes of said type are relatively complicated in terms of infrastructure and setup, which has disadvantageous implications in relation to process implementation and investment costs.

SUMMARY OF INVENTION

The object of the invention is to describe an energy-saving option for removing harmful substances in power stations which are substantially fired by fossil fuels. It is furthermore intended to disclose an easy-to-use apparatus for performing the process.

The invention is based on the fact that in the case of a gas mixture which in large part consists of carbon dioxide CO₂ and includes proportions of valuable gases and harmful substances, particularly in the case of condensation of the carbon dioxide, the harmful substances preferably accumulate in the liquid carbon dioxide. This knowledge is exploited to the effect that the separation of the harmful substances from the liquid phase of the carbon dioxide takes place at low temperatures through the use of adsorber materials/adsorbents, preferably solid adsorber materials. In this case it is particularly advantageous that the energy balance associated with separating harmful substances at low temperatures turns out to be positive, in other words the overall energy requirement is less.

The separation of the harmful substances from the liquid phase can take place at low temperatures very advantageously by means of adsorber materials because the materials employed here have large surface areas which are available for the absorption of the harmful substances.

If the process for separating harmful substances from a gas stream consisting substantially of carbon dioxide is combined with a process in which carbon dioxide is already present in liquid form, synergistic effects in the overall energy balance can thereby be achieved.

It is particularly advantageous to set and maintain the process temperature at less than −30° C. It is not beneficial to use temperatures below −70° C. because carbon dioxide exists in the solid phase at such low temperatures.

The process pressure must in principle lie above the triple point of carbon dioxide in accordance with the temperature/pressure diagram. It is thus at least 5 bar. A temperature range which can likewise be used for the process temperature begins at −5° C. and extends toward lower temperatures.

The use of the process for separating harmful substances from a gas mixture which substantially consists of carbon dioxide can be implemented particularly advantageously in so-called zero-carbon dioxide power stations. In said power stations the separation of the carbon dioxide can be carried out cryogenically. With this approach the carbon dioxide is brought to a low temperature, liquefied and separated out. The liquid phase is very well suited for the adsorptive separation of harmful substances because the latter by preference accumulate in the liquid carbon dioxide. Furthermore, low temperatures promote adsorption, which means for example that a gas scrubbing stage as provided in the prior art is no longer necessary. The use of fixed-bed adsorbers for the adsorption of harmful substances is advantageous because solids having a large surface area are employed here. These are in particular alumina (aluminum oxide), activated carbon, silica gel, zeolites or polymers having a large surface area.

Since the surface area of the adsorbents becomes populated with harmful substances in the course of the process, a periodic regeneration of individual fixed-bed adsorbers is advantageously provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described in the following with reference to schematic accompanying figures, in which specifically:

FIG. 1 schematically illustrates a fixed-bed adsorber through which liquid carbon dioxide containing harmful substances is fed, the harmful substances being adsorbed in the fixed-bed adsorber.

FIG. 2 illustrates adsorbers connected in parallel which can be switched over individually or in groups between the adsorption and regeneration operating states by means of corresponding valves.

DETAILED DESCRIPTION OF INVENTION

The adsorption of harmful substances is carried out in fixed-bed adsorbers, such as are illustrated in FIGS. 1 and 2. After a certain period of use it is necessary to regenerate the adsorbents. This is effected by means of a reduction in pressure, an increase in temperature, or by feeding through a gas or vapor, or by a combination thereof. For this reason there exist a plurality of adsorbers which are in operation or can be regenerated on an alternating basis. An arrangement of three fixed-bed adsorbers is illustrated in FIG. 2. Analogous configurations for two or more than three adsorbers are possible.

During the carbon dioxide condensation a substantial accumulation of harmful substances contained in a gas mixture occurs automatically in the liquid carbon dioxide. It has been possible to demonstrate this with the aid of simulation calculations taking as example hydrogen sulfide H₂S and also carbonyl sulfide COS. The harmful substances can subsequently be separated out from the liquid phase at low temperatures by means of adsorber materials.

The energy-saving separation of the harmful substances at lower temperatures in the overall concept is advantageous. A process according to the invention can be excellently combined with a power station concept which already provides cryogenic carbon dioxide separation.

The temperature range for operating the process can lie between −5° C. and −70° C. The lower temperature limit lies in the region of the solidification of carbon dioxide from the liquid phase, which means that solids can block the process. As a general rule it is necessary to ensure that the fixed-bed adsorbers are kept open for the liquid reaction stream. However, the energy balance of the entire process reveals that temperatures of −30° C. and lower are advantageous for the separation of harmful substances.

The valuable gases such as hydrogen, carbon monoxide, nitrogen or noble gases which may be present in the gaseous or liquid stream in the process are not adsorbed in the course of the described process approach. Harmful substances such as mercury, sulfur or their compounds, which are to be found substantially in the liquid carbon dioxide stream, accumulate on the surface in the fixed-bed adsorbers, in other words are adsorbed by the adsorbents, and are thus retained in the fixed-bed adsorbers. A further harmful substance which can be separated out in this manner is carbonyl sulfide COS.

FIG. 1 shows a fixed-bed adsorber 1 into which a liquid carbon dioxide stream 2 containing harmful substances is introduced. A liquid carbon dioxide stream 3 free of harmful substances emerges at the outlet of the fixed-bed adsorber 1.

FIG. 2 shows three fixed-bed adsorbers 1 connected in parallel. The liquid carbon dioxide stream 2 is supplied at the top, harmful substances are captured in the fixed-bed adsorbers 1, and the liquid carbon dioxide stream 3 free of harmful substances can be removed at the bottom. Individual fixed-bed adsorbers 1 can be switched over in each case for regeneration purposes 4.

While a direct catalytic conversion may be associated with a favorable state of equilibrium on account of the low temperatures, it does however exhibit very unfavorable kinetics. Very large quantities of catalysts would be required, which would probably become very rapidly deactivated due to the presence of sulfur. 

1-8. (canceled)
 9. A process for separating a harmful substance from a gas mixture which essentially consists of carbon dioxide as well as a recyclable substance, comprising: carrying out a carbon dioxide condensation in order to produce and to separate liquid carbon dioxide; carrying out an adsorptive separation of the harmful substance from the liquid carbon dioxide in order to remove the harmful substance from the liquid carbon dioxide; and setting a process temperature in a range from −30° C. to −70° C.
 10. The process as claimed in claim 9, wherein the recyclable substance contained in the gas mixture is selected from the group consisting of hydrogen, carbon monoxide, nitrogen, and a noble gas.
 11. The process as claimed in claim 9, wherein the harmful substance contained in the gas mixture is selected from the group consisting of sulfur, mercury, sulfur compounds and mercury compounds.
 12. The process as claimed in claim 9, wherein that at least one substance from a group of substances comprising activated carbon, zeolite, silica gel, alumina or at least one polymer having a large surface area is selected as an adsorbent for the adsorptive separation of the harmful substance.
 13. The process as claimed in claim 9, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low- or zero-carbon dioxide power stations.
 14. An apparatus for separating a harmful substance from a gas mixture which essentially consists of carbon dioxide as well as a recyclable substance, comprising: a carbon dioxide condensation unit for producing liquid carbon dioxide; and a fixed-bed adsorber for the adsorptive separation of the harmful substance from liquid carbon dioxide for the purpose of removing the harmful substances therefrom, and wherein the operating temperature of the apparatus lies in a range from −70° C. to −30° C.
 15. The apparatus as claimed in claim 14, wherein for the purpose of the adsorptive separation, the fixed-bed adsorber includes at least one adsorbent selected from the group consisting of activated carbon, zeolite, alumina and a polymer having a large surface area.
 16. The apparatus as claimed in claim 14, wherein a plurality of fixed-bed adsorbers are present, and wherein the plurality of fixed-bed adsorbers may be connected in parallel in groups, such that adsorbers or groups of adsorbers may be switched over to allow regeneration of the adsorbents.
 17. The apparatus as claimed in claim 14, wherein the recyclable substance is selected from the group consisting of hydrogen, carbon monoxide, nitrogen, and a noble gas.
 18. The apparatus as claimed in claim 14, wherein the harmful substance is selected from the group consisting of mercury, sulfur, and compounds thereof. 